The method of forming shaped articles from particulate materials is well known in the art. Classically, a desired particulate material is mixed with a fugitive binder and then formed into the desired configuration, this frequently being called the green body. After the forming of the shaped article by any of the methods of the prior art, it is necessary to remove the binder before the green body can be sintered. This procedure is very critical and usually very time consuming which introduces great constraints on the physical and economical viability of the process.
A major drawback with prior art methods is the need for unusual, impractical, uneconomic or unsafe conditions such as vacuum or solvent atmospheres, packing or placement of the articles in or upon absorptive materials, pressurization of the processing vessel, the need for complex multiple thermoplastic binder components with increasing melting points, the generation of internal pressures from gaseous decomposition products and/or the swelling of solvent-binder combinations which tend to disrupt the integrity of the green body and therefore requires extremely flat temperature profiles, the possibility of violent exothermic reactions within the binder which may rapidly destroy the green body and which, in addition, represent a serious danger to the processing equipment, its operators and the environment in general.
In U.S. Pat. No. 4,534,936, Carlstrom et al. recognize the heterogeneity and/or autogeneity of the decomposition reactions of polymeric binding agents. The patent teaches reduction of the time dependence of the reactions by subjecting the rate of temperature rise to a constant arbitrarily or empirically predetermined mass-time derivative, and has therefore achieved a significant reduction in extraction time. The method nevertheless is not thermo-dynamically optimized and furthermore introduces practical problems in the need for accurate initial determination of the net relative mass (weight) of binder in each processing batch, and the incorporation and exposure of sensitive and delicate gravimetric balances to the damaging environment of the ovens or furnaces.
Commonly used organic binders for molding of metal and ceramic particulates generally include various polymeric ingredients, either thermoplastic or thermosetting. Such ingredients may include but are not limited to polyethylene, polypropylene, polystyrene, acrylic resins, methyl cellulose, waxes, paraffins, and the like.
Prior art methodology for the extraction of such binders from green bodies generally involves the use of heat, often in conjunction with chemical leaching agents. Examples of such methods are thermal decomposition, solvation, evaporation, melt-wicking and the like.
These methods often take up much time since the rate of extraction of binder depends primarily on the thickness of the green body in an inverse linear relationship, i.e. the thicker the cross-section of the green body the longer it will take to extract the binder. Also, the rate of binder extraction is limited as a result of: (1) the generation of gaseous decomposition products and/or tensile forces throughout the binder (swelling), tending to disrupt the integrity of the green body, and (2) reduced extraction efficiency as degradation products build up within the green body or in the condensed solvent vapor or liquid solvent. The result is often the introduction of practical constraints for part wall thickness in order to keep the process economical as well as very flat temperature profiles.
It is therefore an object of this invention to provide an improved method of forming articles from sinterable materials. An equally important object of this invention is to provide an improved method for removing organic binder from a molded or compacted body from sinterable materials. It is a yet further object of this invention to provide a self-adjusting thermo-dynamically optimized and therefore energy-saving method for removing organic binder from a molded or compacted body from sinterable materials.